In conventional stereo audio systems, the perceived sound image projected from a pair of loudspeakers can be enhanced or widened by boosting the ambience signal. It is well known that the ambience signal can be obtained from the difference between the left (L) and the right (R) signals of the original source audio. As a result, an audience can feel the ambience signals when they arrive at the right and left ears with different phase shifts. A number of systems have applied this phenomenon to widen the spatial effect in stereo audio systems. These findings are useful in reconstructing enhanced stereo sounds with closely spaced loudspeakers.
A number of improvements have also been made by other systems taking into account of the frequency spectrum and the amplitude of the audio signals that are generated. While there is little doubt about the effectiveness of these techniques, which are well formulated and evaluated, the impressions created on different individuals, and with different kinds of audio, can be diverse. For instance, some audiences may feel weaker sound along certain direction(s). Moreover, stereo signals which have a smaller difference between the left and right channels will generally produce a much weaker enhancement effect. Other factors such as discrepancy in the loudspeakers and listening positions may also play a part in a practical situation.
However, the above mentioned problems may not be obvious when an audience is placed in some real world listening environments, even if he or she is presented with the same stereo signals. One reason for this is that because the ambience signals derived from a dynamic listening environment may be comprised of a complicated juxtaposition of both static and moving ingredients. For example, the movement of objects and people can affect the reflection and reverberation of the acoustic signals. Similarly, movement of the musicians or the musical instruments will often change the impression on the sources of these signals. Accordingly, it is difficult, if not impossible, to impart the desired impressions on the listeners with a spatial enhancement method that processes the source stereo signals in a stationary and deterministic manner.